1. Technical Field of the Invention
This invention relates generally to wireless communication systems and more particularly to radio frequency (RF) transmitters used within such wireless communication systems.
2. Description of Related Art
Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel or channels (e.g., one of the plurality of radio frequency (RF) carriers of the wireless communication system) and communicate over that channel(s). For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the Internet, and/or via some other wide area network.
For each wireless communication device to participate in wireless communications, it includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.). As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage. The low noise amplifier receives inbound RF signals via the antenna and amplifies them. The one or more intermediate frequency stages mix the amplified RF signals with one or more local oscillations to convert the amplified RF signals into baseband signals or intermediate frequency (IF) signals. The filtering stage filters the baseband signals or the IF signals to attenuate unwanted out of band signals to produce filtered signals. The data recovery stage recovers raw data from the filtered signals in accordance with the particular wireless communication standard.
As is also known, the transmitter includes a data modulation stage, one or more intermediate frequency stages, and a power amplifier. The data modulation stage converts raw data into baseband signals in accordance with the particular wireless communication standard. The one or more intermediate frequency stages mix the baseband signals with one or more local oscillations to produce RF signals. The power amplifier amplifies the RF signals prior to transmission via an antenna.
FIG. 1 illustrates a frequency shift keying (FSK) based transmitter of the prior art. The transmitter includes a digital sine wave generator that may be implemented utilizing a direct digital frequency synthesizer (DDFS), digital to analog converters, low pass filters, mixers, a summing module, and a power amplifier. The digital sine wave generator receives digital input data, filters the data using a digital Gaussian low pass filter that is clocked at 24 megahertz, and generates a digital in-phase component and a digital quadrature component based on the filtered data. The instantaneous frequency of the in-phase and quadrature components of the modulation frequency, which is the frequency deviation for FSK (frequency shift keying) modulation, is denoted as ωd. For example, as shown in FIG. 2, when the digital input data is a logic 1, the digital sine wave generator produces a digital cosine wave at its 1st output and a digital sine wave at its 2nd output. The 1st output is processed via a digital to analog converter and a low pass filter and then mixed via a mixer with a cosine signal having a frequency at the radio frequency, i.e., cos (ωRF)t, which corresponds to a in-phase component of a local oscillation. The 2nd output of the digital sine wave generator is processed by another digital to analog converter and another low pass filter and mixed with a sine wave having a frequency at the radio frequency, i.e., sin (ωRF)t, which corresponds to a quadrature component of a local oscillation. As shown in FIG. 2, the output of the mixers are summed, producing a cosine waveform having a frequency that is the sum of the local oscillation (ωRF) and the modulating frequency (ωd). For example, for FSK modulation as used in a Bluetooth application, the modulating frequency is 166 kilohertz.
When the digital input data is a logic 0, the digital sine wave generator produces a cosine wave on its 1st output and a negative sine wave on its 2nd output. These outputs are processed by the respective digital to analog converters and low pass filters and presented as analog sine and cosine waveforms to the mixers. FIG. 3 illustrates the mixing of the cosine wave of the data with a cosine wave of the local oscillation and the mixing of the negative sine wave of the data with the sine wave of the local oscillation. The outputs of the mixers are summed producing a cosine wave that has a frequency that is the radio frequency (ωRF) minus the modulating frequency (ωd). As such, for a digital input of 1, the resulting radio frequency signal is the local oscillation (ωRF) plus the modulating frequency (ωd) and for a logic 0 the resulting frequency is the radio frequency (ωRF) minus the modulating frequency (ωd). Thus, for an FSK Bluetooth application, a logic 1 is represented by a cosine wave having its instantaneous frequency equal to the radio frequency plus 166 kilohertz and a logic 0 is represented by a cosine wave having its instantaneous frequency equal to the radio frequency minus 166 kilohertz.
Such an FSK based transmitter generates a DC offset, which yields local oscillation (LO) leakage that is in band for the RF transmission. Thus, when a receiver receives the RF signal, it also receives the LO leakage. As such, the receiver processes the LO leakage along with the RF signal. If the LO leakage is small with respect to the RF signal, it has little adverse affect on the accurate recovery of data from the RF signals. As the magnitude of the LO leakage increases with respect to the RF signals, its presence decreases the receiver's ability to accurately recapture data from the RF signals.
FIG. 4 illustrates a portion of the FSK based transmitter in greater detail to illustrate how the LO leakage is created. As shown, one output of the digital sine wave generator is converted to an analog current signal by a differential digital to analog converter. The outputs of the digital to analog converter are coupled to ground, or a reference potential, via resistors R1 and R2 and to input resistors R3 and R4 of the low pass filter. Resistors R1 and R2 function to convert the current based output of the digital to analog converter into voltage signals. The low pass filter includes resistors R3-R6 and capacitors C1 and C2 to perform differential low pass filtering. The differential output of the low pass filter is provided to the differential inputs of the mixer. As shown, the mixer mixes the differential output of the low pass filter with a differential local oscillation (e.g., cos(ωRF)t or sin(ωRF)t). In general, the FSK transmitter of FIG. 4 converts the data signals from current signals to voltage signals back to current signals within the mixer. Mismatches between R1 and R2, R5/R3 and R6/R4 cause a DC offset to exist in the differential signal provided to the mixer. The DC offset is further increased by mismatches in the input transistors of the mixer.
FIGS. 5 and 6 illustrate the local oscillation (LO) leakage that is created as a result of the DC offset produced by the mismatches in the current to voltage conversion and within the low pass filter. As shown, the LO leakage appears at the radio frequency. If the DC offset is minimal, the magnitude of the LO leakage is relatively small with respect to the magnitude of the desired RF signal (RF−d or RF+d). However, in many applications, the LO leakage produced by the mismatches between R1 and R2 and the mismatches of the components within the low pass filter is too large.
Therefore, a need exists for a method and apparatus that reduces DC offset within a FSK base transmitter thus producing the resulting LO leakage.